The overall goal of this program project is to identify the cellular and molecular triggers that initiate fatal[unreadable] cardiac arrhythmias and to determine novel molecular-targeted therapeutic strategies to treat them. The[unreadable] central hypothesis is that sudden cardiac death (SCO) can be caused by disruption of molecular complexes[unreadable] and processes that, in normal hearts, underlie balanced regulation of cellular activity and furthermore that[unreadable] altered cellular calcium (Ca) homeostasis plays a critical role in triggering the resulting arrhythmic activity. It[unreadable] is thus the fundamental assumption of this program that, to understand the mechanistic basis of the events[unreadable] that underlie SCO, an integrative approach is necessary that considers variants of ion channel molecular[unreadable] complexes that coordinate the function of intracellular (Project 1) and surface (sarcolemmal) (Project 2) ion[unreadable] channels as well as a critical and central role played by intracellular Ca regulation (Project 3). A key aspect[unreadable] of this Program is the Animal Core B that will maintain and provide genetic mouse models to each of the[unreadable] three Pis. This project (Project 1) focuses on elucidating defects in the regulation of type 2 ryanodine[unreadable] receptor (RyR2)/SR Ca release channel, which is required for cardiac excitation-contraction (EC) coupling.[unreadable] We propose to test the hypothesis that a diastolic SR Ca "leak" via defective RyR2 can serve as triggers for[unreadable] fatal cardiac arrhythmias which cause sudden cardiac death (SCO). A new theme that has emerged from this[unreadable] project is the concept that there are genes that encode proteins which are "protective" against SCO. During[unreadable] the previous funding period of this project the applicant identified a defect in RyR2 in patients with exercise-induced[unreadable] SCO: "leaky" RyR2 channels that exhibit decreased binding affinity for the stabilizing subunit[unreadable] calstabin2 (FKBP12.6). This observation has directly lead to the development of a novel potential therapy[unreadable] for SCO, RyCal (JTV-S36, which is a JTV-519 derivative without HERG and L-type channel blocking[unreadable] activities), representing a new class of intracellular calcium channel stabilizers that prevent exercise-induced[unreadable] SCO in mouse models. During the previous funding period the applicant identified the phosphodiesterase[unreadable] PDE4D3 as a novel component of the RyR2 macromolecular signaling complex and preliminary data,[unreadable] presented in this proposal, suggest that PDE4D3 in the RyR2 complex may be "protective" against SCO[unreadable] because mice that are deficient in PDE4D3 are predisposed to exercise-induced SCO. Moreover, a mouse[unreadable] engineered to express an RyR2 channel that cannot be PKA phosphorylated, RyR2-S2808A, is protected[unreadable] against SCD induced by inhibition of PDE4 with rolipram. Close interactions with the Pis of Project 2 (R.[unreadable] Kass - voltage-gated ion channels) and Project 3 (W.J. Lederer - calcium signaling) enhances each of the[unreadable] proposed aims as detailed below. Thus this work has the potential to determine a mechanistic basis for[unreadable] Sudden Cardiac Death (SCD) at the molecular level, and to develop therapeutic strategies in man based on[unreadable] specific molecular and mechanistic models.